In the semiconductor manufacturing world, manufacturers produce plasma processing chambers that utilize radio frequency (RF) power to generate a plasma. In order to achieve efficient power transfer between the RF generator (“generator”) and the plasma load, an impedance-matching network (“match”) is often used to match the load impedance to a desired input impedance, typically 50 ohm. Plasma load impedance may vary depending on variables such as generator frequency, power, chamber pressure, gas composition, and plasma ignition. The match accounts for these variations in load impedance by varying electrical elements, typically vacuum variable capacitors, internal to the match to maintain the desired input impedance.
Match networks typically contain reactance elements, meaning elements that store energy in electrical and magnetic fields as opposed to resistive elements that dissipate electrical power. The most common reactance elements are capacitors, inductors and coupled inductors but others such as distributed circuits are also used. Match networks can also include lossless elements including transmission lines and transformers. The only resistive elements in a match network are typically associated with losses in non-ideal reactive and lossless components or components that do not take part in the impedance transformation such as components for sensing voltage, current, power or temperature.
Match networks can comprise a number of variable reactance elements. For instance, vacuum variable capacitors can be used. However, these are bulky and expensive. In the alternative, banks of parallel capacitors having different capacitances, and being added or removed from the parallel circuit via electrical switches have also been considered. Often, such capacitor banks use high power PIN diodes (controlled by a transistor) to switch the capacitors in and out of the parallel system. However, such PIN diodes can be too slow for RF power applications, or can require excessive power to accomplish the switching at acceptable speeds. This in turn results in running the PIN diodes at high temperatures. PIN diodes are also expensive and only produced by a handful of manufacturers.